# Sound Waves

1. Dec 6, 2009

### Dark Visitor

A sound wave in air has a frequency of 500 Hz and a wavelength of .68 m. What is the air temperature?

* -18 degrees C
* 0 degrees C
* 15 degrees C
* 27 degrees C

2. Dec 6, 2009

### denverdoc

Try googling for speed of sound. It varies with temp. So compute the speed from the data and compare with what you find online. Google is your friend.

3. Dec 6, 2009

### Dark Visitor

Not that your suggestion is wrong, but when looking in my book, it says "use the speed of sound in air at 20 degrees C, 343 m/s, unless otherwise specified."

So I guess I use that. But where does that help?

4. Dec 6, 2009

### denverdoc

The velocity of C=lambda*f. You are given both above--so what is C. It is not 343. Still do a google, meanwhile do you think the speed of sound increases or decreses with temp?

EDIT: I'll spare you the math as its been a long day: here is a table: http://www.sengpielaudio.com/calculator-speedsound.htm

5. Dec 6, 2009

### Dark Visitor

When I Googled it, the first thing that popped up was "speed of sound is 340.29."

And then if I multiply:

500 Hz * .68 m, I get 340. How does that connect?

6. Dec 6, 2009

### denverdoc

Go back to my earlier post, I edited it, and remember google is your friend. There are lots of time when animations and what not can make a difficult concept clear. Books cannot do this!

7. Dec 6, 2009

### Dark Visitor

Thanks for the link, but what do I type in? I don't know a temperature.

8. Dec 6, 2009

### denverdoc

Find the temperature that corresponds to 340m/s and compare with your answer choices. Scroll down to the bottom of the page and find TABLE--I see your confusion.

9. Dec 6, 2009

### Dark Visitor

Okay. I did that, and 15 degrees C is the closest (it was 340.27). But how would I show my work for that? Unfortunately, my teacher would not accept "I used Google to get my answer" as an acceptable excuse. :uhh:

10. Dec 6, 2009

### denverdoc

Well, then look up the math. I don't know what you're using for a book or your notes, but there are several formula of increasing complexity that allow you to calculate C as a function of T. If your book says use 343, I'm assuming there is no discussion in it re variability. Wiki has a decent discussion with two fairly simple equations uou can solve in a couple of minutes.

11. Dec 6, 2009

### Dark Visitor

I don't see any way to show any math, so I just put a note on the side saying (343 is the speed of sound in 20 degrees C).

12. Dec 6, 2009

### denverdoc

As i said, its on wikipedia, or any of a 1000 other sites. But your call.BTW look at wiki under practical formulas for dry air.

C=331+0.6*c where c=temp in degrees c. Plug in 340 and you'll get 15 degrees c!

13. Dec 6, 2009

### Dark Visitor

I know, but I don't know which one to use, or how to use it. I'll keep looking. Can you finish helping me on "Find The Wavelength" please? It doesn't have to be tonight, but I need all my posts done by tomorrow and I have 3 left, including that one. Or help me on any others (the other 2 are "Kinetic Energy at the Bottom" and I am gonna post another one in a second.) Thanks so much.

14. Dec 6, 2009

### denverdoc

look at the formula above on my last post. That is very simple.